Feature Request: Add verify_keyless_github_actions function

[viccuad]

Is your feature request related to a problem?

We want to implement kubewarden/policy-fetcher#91. This is a breaking change, that will break https://github.com/kubewarden/verify-image-signatures if configured to to verify images sign via keyless in GHA.

Solution you'd like

Add a new verify_keyless_github_actions function that uses githubActionsVerifier instead of GenericIssuerSubjectVerifier to validate keyless signatures done in GHA.

We can consume that new verify_keyless_github_actions function in https://github.com/kubewarden/verify-image-signatures then.

Alternatives you've considered

If we have easyjson on the Go SDK, maybe it's simpler to provide a callback that accepts a full VerificationConfig instead.

Anything else?

No response